Cordierite (Mg2[Al4Si5O18]) ceramics are the preferred materials for use in automotive catalytic substrates, diesel particulate filter applications, and other high temperature articles, such as NOx adsorber substrates, catalyst substrates, and honeycomb articles due to the combination of their low cost of production and physical properties such as low coefficient of thermal expansion (CTE) and resistance to thermal shock.
Cordierite substrates are typically produced from naturally occurring minerals such as talc and kaolin due to their lower cost and high purity. Cordierite materials are typically manufactured by mixing a raw batch that includes talc, alumina, aluminum hydroxide, kaolin and silica. The batch is then blended with a binder (such as methylcellulose) and a lubricant (such as sodium stearate) to form a plastic mixture. This plastic mixture is then formed into a green body and sintered.
The cordierite crystal structure consists of a hexagonal ring of tetrahedra that are joined at each intersection of the hexagonal ring by five silicon and one aluminum atom. The hexagonal rings are connected together by additional aluminum tetrahedral and magnesium octhedra resulting in two interstitial vacancies per unit cell that are oriented along the c-axis of the crystal structure. See, B. P. Saha, R. Johnson, I. Ganesh, G. V. N. Rao, S. Bhattacharjee, T. R. Mahajan; Materials Chemistry and Physics, 67 (2001), 140-145. The interstitial vacancies result in a contraction along the c-axis of the crystal structure and an expansion along the a- and b-axes with increasing temperature. See, R. J. Beals, R. L. Cook, J. Am. Ceram. Soc., 35(2), (1952), 53-57. The anisotropic CTE resulting from the cordierite crystal structure offers the opportunity to engineer improved cordierite honeycombs by orienting the c-axis of the individual crystals within the Ceramic in the direction of extrusion. Cordierite crystal orientation has been observed to cause a significant net decrease in the overall CTE of the ceramic honeycomb. See, I. M. Lachman, R. M. Lewis, U.S. Pat. No. 3,885,977, May 27, 1975; and R. Johnson, I. Ganesh, B. P. Saha, G. V. Narasimha Rao, Y. R. Mahajan, J. Mater. Sci., 38 (2003), 2953-61.
In order to orient the cordierite crystals within the ceramic, platy raw materials are used. In particular, talc and kaolin have platy crystal structures that may be preferentially oriented parallel to the direction of extrusion when passed through an extrusion die at high pressure. Delamination of hydrous kaolin may be utilized to increase the platyness of the clay increasing alignment during extrusion. Subsequent sintering of the green body results in the formation of a ceramic with preferential orientation of cordierite crystals within the honeycomb structure oriented along the c-axis relative to the extrusion direction. See, I. M. Lachman et al., U.S. Pat. No. 4,772,580, Sep. 20, 1988. Although talc and kaolin both play a role in orienting the sintered cordierite crystal structure, kaolin is considered to be the most significant contributor because it provides the only source of ordered aluminum within the green body. Since silicon comes from both talc and kaolin raw material sources and magnesium (talc as the source) makes up a smaller atomic and weight percent of the final cordierite crystal, aluminum (derived from kaolin) is expected to have the greatest contribution to the final cordierite crystal structure. See, Saha et al.
One drawback with producing a highly ordered cordierite substrate is that the difference in thermal expansion along the axial and transverse directions in the honeycomb becomes so large that cracking occurs resulting in reduced thermal shock resistance. See, Saha et al. Although this is a concern for catalyst substrates, it is of particular significance to honeycombs produced for diesel particulate filter applications where increased porosity lowers the shock resistance of the resulting ceramic. In addition, the extrusion of highly oriented raw materials parallel to the axial direction of the substrate lowers the strength of the green body resulting in sagging of the body, particularly in thin wall applications. To alleviate these problems, calcined clay is typically added in combination with delaminated hydrous clay to moderate particle alignment within the green body and subsequent cordierite crystal alignment within the sintered ceramic. Calcination produces a coarser particle that is less platy in nature particularly compared to delaminated hydrous clay.